It has been common practice to apply aqueous coatings, consisting of pigments, binders/adhesives, and various functional additives, to paper to produce printing paper. The coatings cover the fibrous paper sheet surface and create an interface for printing inks that is smoother and less absorbent than the uncoated surface.
Prior to coating, the uncoated paper web exhibits many "micro" depressions. The topography of the micro depressions can be seen in FIG. 1. The micro depressions are often the result of open areas between individual fibers and fiber bundles in the paper. The dimensions of these open areas determine the roughness of the paper surface and the amount of coating required to completely fill in the depressions. The micro depressions on the rough sheet surface can vary in depth from as little as 5 microns to as much as 25 microns (1 micron=1.times.10.sup.-6 inch). The average depth of the micro depressions is 10 microns.
The bulk-to-weight ratio of typical coating layers in the prior art is approximately 1.00 micron per lb. of coating of applied, per side, per ream of paper. As used herein a "ream" equals approximately 3,300 ft.sup.2 of paper. Application of one lb. of conventional coating per side, per ream of paper will generally result in a layer of coating on the surface of the paper that is 1 micron thick.
FIG. 2 illustrates the topography of a typical paper sheet surface when a coating is applied to the surface by a blade coater, a coater commonly used for applying aqueous coatings for the production of printing paper. To completely fill in the micro depressions on the surface of a paper web, prior art processes will apply 5-25 lbs. of coating per side, per ream of paper web. The amount of coating needed will depend on the formation of the paper web, its absorbent properties, and on the roughness of the particular paper web. For instance, 25 lbs. of coating per side, per ream of paper web may be needed to completely fill in the micro depressions on the surface of a very rough, absorbent paper web.
Although greatly improving print properties, the amount of coating used in coated papers of the prior art significantly alters the appearance of the paper. Coated papers, whether glossy or matte in finish, have an appearance that is much different than that of uncoated premium papers, such as text and cover paper. This difference is considered undesirable by many paper consumers who prefer the appearance of uncoated paper but need the printability properties of coated paper.
Uncoated paper typically can be lightly coated, but the amount of coating is less than 5 lbs. per side, per ream of paper. This lower amount of coating does not alter the desirable uncoated appearance of the paper. However, prior art uncoated papers of this type have never achieved the printability properties of coated paper. Thus, there is great interest in the development of a printing paper which combines the desirable appearance of uncoated paper with the printability properties of coated paper.
Several techniques have been developed in attempts to utilize existing size press equipment to manufacture coated paper. Size presses are typically used in the art to make uncoated paper and are normally used to apply coatings having low viscosities of less than 100 centepoise ("c.p.s.") (Brookfield, 100 r.p.m.). One such technique, known as "film coating" or "wash coating," utilizes existing size press equipment to achieve a slight improvement in the printing paper over uncoated offset paper grades without the capital expense of normal coating treatments. The resulting paper is, however, a lower quality coated paper and often exhibits undesirable characteristics, such as mottle (galvanized gloss) and roll film split pattern.
Another technique involves the use of micro-spherical particle pigment, which has been used to promote gloss and opacity in coated paper. For instance, Kowalski et al. (U.S. Pat. No. 4,427,836) relates to a process for manufacturing water-insoluble, heteropolymer, micro-spherical particles, which may be used as an opacifier or thickener when incorporated in a paint or on aqueous coating.
Vassiliades, et al. (U.S. Pat. Nos. 3,816,169 and 3,822,181) disclose spherical microcapsules that may be used as an opacifier when incorporated in or coated on a paper web. These patents concern the opacifying and/or brightening of the paper web with no regard for printability. The patents do not disclose any specific formulations that could be used to coat the paper. Nor do the patents discuss how such formulations could be applied.
Rohm and Haas Company manufactures Ropaque.RTM., a pigment for use on paper, which is composed of polymeric, hollow, spherical particles ranging in diameter from 0.3-1.0 micron. These particles enhance printability of paper and have a high bulking capability. The particles may be combined with typical binders, such as starch binders, and other ingredients, to create coatings having a greater thickness at a lower weight than typical coatings. When these coatings are applied at low viscosity (less than 100 c.p.s., Brookfield, 100 r.p.m.) using the "wash" or "film" coating techniques described above, the resultant paper exhibits a marginal improvement in printability and opacity.